Sunday, September 14, 2014

On-chip decoupling capacitors

In virtually all of my micro-controller projects, I'll use 0.1uF ceramic capacitors between Vcc and Gnd.  Depending on the power draw of the MCU and inductance on the power lines, they may not be necessary, but at a cost of a penny or less each there's little reason not to use them.

I remembered seeing CPUs that have on-chip decoupling capacitors, and thought it would be nice if the MCUs I'm using had the same.  When working with small projects on mini breadboards, not having to find space for the decoupling cap would be convenient.  It would also save me the trouble of digging through my disorganized collection of components looking for that extra capacitor.

My first idea was to glue a 0805 (2mm x 1.25mm) MLCC to the top of the chip, and then solder 30AWG wire-wrap to the power and ground leads.  I used contact cement, and although it seemed secure after drying for about 30 minutes, once I added flux and touched it with my soldering iron tip it moved freely.  Then I tried a small drop of super glue, but for some reason it wasn't dry after an hour; maybe it was defective.  If someone knows of a glue that would work well, let me know in the comments or send me an email.

Even after drying for a day, neither the contact cement nor the super glue would securely hold the capacitors while I tried to solder them.  With the help of a pair of tweezers I was able to solder a MLCC to the top of an ATtiny85 as shown in the photo above.

For 28-pin DIP AVR MCUs that have ground and power on adjacent pins, the job is a lot easier.  Here's a ATtiny88-PU with a 0805 MLCC:

The easiest method I came up with doesn't require any glue.  I trimmed, then soldered the leads of a ceramic disc capacitor to the power and ground pins of an ATtiny84a:

2014/10/20 Update:
I found information about a high-temperature component adhesive which indicates typical cyanoacrylate adhesive is stable only up to 82C.  For something that is readily available in hardware stores, I may try silicone adhesive, or even BBQ paint.


  1. Have you tried just normal dime-store JB weld? Rated for a max temp of 600F. Maybe just a dab on the belly of the component. Sets up fast too.

    1. I picked some 2-part epoxy up today to try. First observation is that in comparison to how much it takes to cover the bottom of a 0805 cap, a lot of the epoxy gets wasted. And trying to squeeze out equal tiny amounts is rather tricky, so if it doesn't work on the first try I may have to mix a larger batch to ensure I'm getting close to 50/50. Gonna let it cure overnight and put the soldering iron to it tomorrow.

  2. AVRs really should include a decoupling cap inside the package right next to the die. That said, AVRs are amazingly tolerant and I've had good luck omitting the cap as long as there is a stiff supply and short wires.

    1. I agree you can usually get away without the cap. However everytime I'm debugging a circuit without a decoupling cap I end up wondering if it could be causing problems.
      The one situation I found where I did need it was when I was using the 3.3V output from a chinese USBASP for powering a circuit. After more investigation, the real problem was the USBASP though, since there was no cap on the output side of the AMS1117 regulator. The easy fix was to solder a 0805 4.7uF cap between 2 of the legs of the AMS1117.